The present invention relates to the manufacture of body implantable medical devices, and more particularly to making stents and other prostheses configured for high radio-opacity as well as favorable mechanical characteristics.
Several prostheses, typically of latticework or open frame construction, have been developed for a variety of medical applications, e.g. intravascular stents for treating stenosis, prostheses for maintaining openings in the urinary tracts, biliary prostheses, esophageal stents, renal stents, and vena cava filters to counter thrombosis. One particularly well accepted device is a self-expanding mesh stent disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The stent is a flexible tubular braided structure formed of helically wound thread elements.
Alternatively, stents and other prostheses can be expandable by plastic deformation, usually by expanding a dilation balloon surrounded by the prosthesis. For example, U.S. Pat. No. 4,733,665 (Palmaz) discloses an intraluminal graft constructed of stainless steel strands, either woven or welded at their intersections with silver.
Regardless of whether the prosthesis is self-expanding or plastically expanded, accurate placement of the prosthesis is important to its effective performance. Accordingly, there is a need to visually perceive the prosthesis as it is being placed within a blood vessel or other body cavity. Further, it is advantageous to visually locate and inspect a previously deployed prosthesis.
Fluoroscopy is the prevailing technique for such visualization, and it requires radio-opacity of the materials to be imaged. The preferred structural materials for prosthesis construction, e.g. stainless steels and cobalt-based alloys, are not highly radiopaque in the thin-section sizes of stent wires. Consequently, endoluminal prostheses constructed of these materials do not lend themselves well to fluoroscopic imaging.
A particularly advantageous stent construction, in terms of providing radio-opacity and mechanical integrity, is disclosed in U.S. Pat. No. 5,630,840 (Mayer). The Mayer device discloses a stent formed of multiple filaments, preferably arranged in at least two sets of oppositely directed helical windings interwoven with one another in a braided configuration. Each filament is a composite including a core surrounded by a case. Preferably, the core provides the desired radio-opacity, while the case governs mechanical behavior. Suitable core materials include tantalum and a platinum nickel alloy. Suitable case materials include certain alloys composed primarily of cobalt and chromium, e.g. sold under the brand names Elgiloy and MP35N.
Although such composite filaments provide the desire d mechanic al characteristics and good fluoroscopic visibility, the primarily platinum core is expensive.
Accordingly, one object of the present invention is to provide a process for making composite filament for use in stents and other body insertable medical devices, that is less expensive than composite filaments with primarily platinum cores.
Tantalum cores are less expensive than platinum cores, but require special processing. During reduction of a tantalum billet or bar to a rod or wire for use as the core, the material is aninealed to enhance its formability. Annealing usually occurs in a hydrogen atmosphere to prevent oxidation of the tantalum. Hydrogen is absorbed by the tantalum during annealing, and must later be removed by a vacuum treatment to avoid hydrogen concent rations in the metal. During the age hardening stage of composite filament construction, any residual hydrogen in the tantalum core can lead to hydrogen outgassing, increasing the time needed to achieve satisfactory vacuum levels within the age hardening furnace for protecting the case material from oxidation.
Accordingly, another object of the present invention is to provide a process for making composite filaments in a manner that avoids hydrogen outgassing during the processing of filaments or stents and other devices composed of the composite filaments.
A further object of the invention is to provide a composite filament fabrication process incorporating cold-working stages and annealing stages performed under conditions more suitable for filaments having cores with melting temperatures lower than those of tantalum and platinum.
Another object is to provide a process for fabricating composite filaments for devices that, when inserted or implanted in the body, are less likely to interfere with magnetic resonance imaging of tissue adjacent and surrounding the devices.
Yet another object of the invention is to provide a process particularly well suited to fabricating a composite filament having a gold or gold alloy core.